Switching to kevlar tiedown cables


 Why do we need tiedowns:

  Most telescope pointing errors are repeatable. These include beam deflections caused by the motion of  the azimuth and dome. Using a pointing model (made by tracking sources), we can get the pointing error of the telescope down to about 5 arc seconds rms. This is needed since the FWHM beam at xband is 25 arcseconds,.

    A 5 arcsecond error is a motion of 3 mm at the feed. The repeatability of the errors allows us to position the horns in absolute space to about 3mm.

    The one pointing error that is not repeatable is temperature. When the main cables heat up, they expand, when they cool off ,they  contract. During the night times, the telescope will move vertically 1 inch for every 5 degree temperature change

    To get the 5 arcsecond pointing rms we need to remove the temperature variability. This is done using laser rangers and an active tiedown system

The tiedown system:

    The tiedown system controls the average height of the platform. The system consists of:

Constraints in using the tiedown system.

Problems with using the tiedown system:

Replacing the tiedown cables with kevlar cables.

    A proposal was made to replace the wire rope tiedown cables with kevlar cables. The benefits of this are:


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